1. Field of the Invention
The invention relates to pistons and more particularly to the pressure casting of pistons with crown inserts and cavities in the casting for the circulation of coolants.
2. Review of the Prior Art
In the conventional gravity casting of pistons, a known method of forming such cavities comprises formation of a soluble core in the shape of the required cavity and then placing the core in a required position in the mould before the piston is cast. Such cores are commonly of a salt such as sodium chloride but may be of any alternative soluble salt or mixture of salts. Where gravity die cast pistons are cast "crown down" (i.e. with the crown lowermost), the salt core can either be connected to the mould core and lowered into the other mould member with the mould core. Alternatively the salt core can be positioned in the lower mould member by the formation of the salt core with integral salt legs which enable the salt core to stand on the base of the lower mould member or, where such is provided, on a crown insert also placed in the lower mould member. After casting the salt is flushed out with a suitable solvent to leave a cavity for the circulation of coolants.
In recent years, attention has been directed to the pressure casting (for example squeeze casting) of pistons. In such a casting technique, the molten piston metal is solidified under pressure to produce a casting whose structure is particularly homogeneous and free from voids and which is therefore stronger than a gravity die casting. In general, in pressure casting, the piston is cast "crown down" in order to ensure that the molten metal first entering the die does not solidify before pressure is fully applied.
The formation of cavities in such pressure cast castings by the use of salt cores is, however, a problem because the methods described above for use in gravity die casting cannot be successfully used. If the salt core is connected to the mould core, there is a possibility both of damage to the salt core and incorrect location of the salt core because the mould members continue to move relatively to one another during solidification of the casting as a result of contraction of the casting under pressure. If the salt core stands on salt legs, the pressure is often sufficient to cause the molten metal to penetrate the salt legs, thus making the salt core difficult to flush out. The pressure may also cause the salt legs to fracture, so allowing movement of the salt core out of position.